The present invention relates to a method for sealing an annulus between tubulars or between a tubular and a borehole.
Conventionally, in order to achieve a seal between a tubular and a borehole, the annulus (the gap between the casing and the rock/formation) is subjected to a cementing (or grouting) operation. This treatment is normally referred to a Primary Cementing. The main aspects of primary cementing are to isolate flow between different reservoirs, to withstand the external and internal pressures acting upon the well by offering structural reinforcement and to prevent corrosion of the steel casing by chemically aggressive fluids.
A poor cementing job can result in migration of reservoir fluids, even leading to gas migration through micro-annuli in the well which not only reduces the cost-effectiveness of the well but may cause a xe2x80x9cblow outxe2x80x9d resulting in considerable damage. Although repair jobs (xe2x80x9csecondary cementingxe2x80x9d) are possible (in essence forcing more cement into the cracks and micro-annuli) they are costly and do not always lead to the desired results.
One of the major drawbacks of the use of traditional cementing materials such as Class G cement (e.g. OPC: Ordinary Portland Cement) is that such materials cannot achieve a gas tight seal due to the inherent shrinkage of the materials. Shrinkage is typically in the order of 4-6% by volume which causes gas migration through the micro-annuli created because of the shrinkage.
It has been proposed in the art to use a mixture of a slurry of a hydraulic cement and a rubber component in order to improve on the ordinary sealing properties of the conventional cementing materials. However, the intrinsic properties of the conventional cementing material still play a part in such sealing techniques.
Cementing can also be carried out between two tubulars, e.g. in order to fix a corroded or damaged pipe or for upgrading the strength of a packed pipe.
A technique known in the oil industry as expansion of well tubulars, normally introduced to complete an uncased section of a borehole in an underground formation, has as one of its features that it narrows the gap between the outer surface of the tubular and the casing and/or rock/formation it faces. However, it is not envisaged and in practice impossible to provide even a small sealing effect during such expansion operation.
In European patent specification 643,794 a method is disclosed for expanding a casing against the wall of an underground borehole wherein the casing is made of a malleable material which preferably is capable of plastic deformation of at least 25% uniaxial strain and the casing may be expanded by an expansion mandrel which is pumped or pushed through the casing. Again, it is not envisaged and in practice impossible to provide even a small sealing operation during such expansion operation.
It is also known in the art that tubulars can be provided with coatings (also referred to as xe2x80x9ccladdingsxe2x80x9d) which are normally applied in order to increase the resistance of the tubulars against the negative impact of drilling fluids and other circulating materials (e.g. fracturing agents or aggressive oil field brines). Again, such provisions are not designed to obtain any improvement with respect to sealing.
Recently, in International Patent Application WO99/02818 a downhole tubing system has been proposed which in essence is based on a radially expandable slotted tubular body carrying deformable material on the exterior thereof and a seal member within the tubular body and for engaging an inner surface of said body. It is specifically stated that there should be, of course, no elastomer-to-rock contact at the positions of the slots as the inflow of oil should not be interrupted.
Therefore, the system as described in WO99/02818 has to be regarded as a system which allows flow of fluid at certain places (envisaged because of the presence of the slots) and not in others which is achieved by the combination of three elements: the use of an expandable tube, the presence of a deformable material on the exterior of the tubular body and the use of a seal member inside the expandable slotted tubular body.
There is no reference in the description of WO99/02818 to expandable solid tubulars.
In recently published International Patent Application WO99/06670 reference is made to a method for creating zonal isolation between the exterior and interior of an uncased section of an underground well system which is located adjacent to a well section in which a casing is present. The zonal isolation is obtained by inserting an expandable tubular through the existing well casing into an uncased section, such as a lateral branch, of the underground well system and subsequently expanding the expandable tubular such that one end is pressed towards the wall of the uncased section of the well system and the outer surface of the other end is pressed against the inner surface of the well thereby creating an interference fit capable of achieving a shear bond and an hydraulic seal between said surrounding surfaces. It is possible to insert a gasket material between the surrounding surfaces before expanding the tubular.
It will be clear that the method proposed in International Patent Application WO99/06670 is aimed particularly at machined tubulars which are rather regular and the hydraulic seals formed are useful because of the concentric nature of the surrounding surfaces.
It has now been realised that under more demanding conditions, in particular when the tubulars or a tubular and borehole are less concentric with respect to each other and may also vary in radial dimensions, providing adequate seals by straight forward expansion, even when using a gasket, is no longer possible. Even systems which were initially well sealed because of the concentric, or substantially concentric nature of the tubulars or the tubular and the borehole, will deteriorate with time due to a variety of circumstances such as corrosion, displacement forces and the like. This means that there is a need to devise a sealing system which can operate under practical conditions and, preferably over rather long distances. Moreover, such sealing system should be capable of performing its sealing duty over a long period of time during which conditions may vary as discussed hereinabove.
A method has now been found which allows the formation of good quality seals when use is made of the expanding feature of an expandable tubular to provide a sealing based on thermoset or thermoplastic material.
The present invention therefore relates to a method for sealing an annulus between two solid tubulars or between a solid tubular and a borehole which comprises the use of a thermoset or thermoplastic material in forming the seal between at least part of the outer surface of a tubular and at least part of the inner surface of the other tubular or the wellbore in which the seal is formed by expanding the inner tubular.